The present invention relates to a wireless telecommunications system. More particularly, the present invention relates to a wireless communication system that is capable of communicating over a local cellular/PCS air interface or, alternatively, a satellite air interface.
Dual-mode satellite/cellular phones are well-known in the art, and they are capable of operating in either a cellular/PCS mode or in a satellite mode. However, such phones are relatively large and heavy compared with ordinary cellular phones. This is especially true, now that cellular technology is producing cellular telephone handsets with a size and weight approximating that of a credit card.
The reason why satellite telephones and dual-mode cellular/satellite phones are relatively large and heavy compared to standard cellular telephones is that the components required to communicate over the satellite air interface are, in general, much larger than those required to operate over the cellular/PCS air interface. For example, the directional antenna in a satellite telephone is large so that it provides the directional gain needed to help overcome the poor link margin that is characteristic of satellite signal transmissions. In addition, satellite telephones and/or dual-mode cellular/satellite phones contain relatively large power amplifiers and batteries, which are also needed to help overcome the relatively poor link margin. The added size and weight due to these components make satellite phones and/or dual-mode phones bulky and cumbersome to carry.
Previous attempts have been made to reduce the size and weight of dual-mode satellite/cellular phones. Some of these efforts have focused on developing fully integrated baseband chip sets that incorporate the functionality of both terrestrial (i.e., land-based cellular) and satellite modes. However, such designs are still relatively bulky and heavy, as many of the above-identified components required to communicate over the satellite air interface are still required outside the baseband portion of the unit.
In U.S. patent application Ser. No. 08/756,709, "Satellite Communications Adapter for a Cellular Handset," some or all of the heavy, power intensive satellite-mode components, such as the directional antenna, the battery and the power amplifier, are maintained in a satellite signal relay device, which is physically separate from the dual-mode satellite/cellular telephone handset (i.e., the man-machine interface). Therefore, the handset itself remains somewhat small and light weight. In the satellite-mode, the dual-mode satellite/cellular handset transmits and receives signals to and from the satellite network through the satellite signal relay device. More specifically, upon receiving signals from the satellite network, the satellite signal relay device downconverts the RF transmission into a low power cellular signal and then re-transmits the signal to the less bulky, lightweight handset over a local cellular frequency. Similarly, upon receiving a satellite communications signal from the handset, the satellite signal relay device upconverts the signal and then re-transmits it to the orbiting satellite network.
There will be situations, however, when it is advantageous to communicate via satellite directly, without first having to transmit and receive over an additional cellular link. As the link margins associated with satellite signals are already inherently low, as mentioned above, a user, under certain conditions, may have difficulty achieving a good quality link if, in addition to the already low link margin associated with the satellite link, the interference levels and distortional effects present in the local cellular environment further degrade the communication signal. Under such conditions, the user may wish to have the option to place or receive a satellite call directly, and to avoid further signal degradation caused by a local cellular/PCS link.
Another consideration is that satellite subscribers expect to have global or near global coverage. Yet dual-mode satellite/cellular systems in which the satellite-mode relies on a cellular/PCS link run the risk of being operationally limited to regions which provide compatible cellular services. For example, a dual-mode, satellite/cellular phone that is compatible with the GSM standard used in Europe could not be used in either a satellite or a cellular mode in the United States, since the United States employs the AMPS standard, as those skilled in the art will recognize. Even if the above limitation could be solved, a local cellular/PCS operator (or regulatory body) may object to having the cellular/PCS frequencies reused for relaying information to the satellite signal relay device, since this may cause, under certain conditions, interference to the local cellular/PCS network.
Clearly, a need exists for a satellite/cellular phone system that allows a user to place either cellular calls or satellite calls through a lightweight satellite/cellular handset. Alternatively, a user must maintain the ability to place satellite calls through a stand-alone (i.e., self-contained) unit, wherein the communication signals are not subject to any incompatibility with local cellular services, nor subject to interference and/or distortion that may be present in the local cellular environment. Furthermore, there is a need to better ensure that the satellite subscriber has truly global or near global capability.